System.Security.Cryptography.CryptographicException: Bad Data.

   at System.Security.Cryptography.CryptographicException.ThrowCryptographicException(Int32 hr)
   at System.Security.Cryptography.Utils._DecryptData(SafeKeyHandle hKey, Byte[] data, Int32 ib, Int32 cb, Byte[]& outputBuffer, Int32 outputOffset, PaddingMode PaddingMode, Boolean fDone)
   at System.Security.Cryptography.CryptoAPITransform.TransformFinalBlock(Byte[] inputBuffer, Int32 inputOffset, Int32 inputCount)
   at System.Security.Cryptography.CryptoStream.FlushFinalBlock()
   at System.Security.Cryptography.CryptoStream.Dispose(Boolean disposing)
   at System.IO.Stream.Close()
   at Web.Site.Utils.Captcha.Cryptography.Decode(String tIn, Int32 tLen) in D:\RadoWork\WWW_TWTCM\twtcm\Web.Site\Utils\Captcha\Cryptography.cs:line 86
   at Web.Site.Utils.Captcha.Cryptography.Decode(String tIn) in D:\RadoWork\WWW_TWTCM\twtcm\Web.Site\Utils\Captcha\Cryptography.cs:line 54
   at Web.Site.Utils.Captcha.HttpHandler.ProcessRequest(HttpContext context) in D:\RadoWork\WWW_TWTCM\twtcm\Web.Site\Utils\Captcha\HttpHandler.cs:line 32